This invention relates to energy-charging type half-cells in which optical energy can be efficiently stored, and the energy thus stored can be efficiently obtained as electrical energy. More particularly it relates to a rechargeable optical energy charging type half-cell in which an energy storage stage can be efficiently obtained by application of light after discharge. The invention also relates to a photochemical battery which is obtained by combining the optical energy charging type half-cell and an oxidation-reduction system half-cell.
Recently, the conversion of sunlight energy into electrical energy has been the subject of many studies. As a result of these studies, a so-called photochemical battery has been provided in accordance with a method in which a photochemical reaction is utilized, and the energy of the resultant chemical change is obtained as electrical energy. Since the photochemical battery is manufactured at a relatively low cost, it is expected to be put in practical use when designs are optimized.
In order to provide a photochemical battery of high performance, the following requirements must be satisfied: (1) the photochemical reaction to be utilized is endothermic with high quantum yield (at least 20 percent) (2) a material produced by the photochemical reaction can be stored so that it can be obtained as electrical energy later on; (3) and after the electrical energy is obtained through discharge, the storage state can be obtained again by applying light (i.e., optical energy recharging is possible). It is advantageous that the energy be stored as compactly as possible. Accordingly, it be desirable that the energy is stored in a solid phase.
One of the photochemical cells which are important in view of the above-described requirements is an optical energy rechargeable galvanic cell which is obtained by combining silver chloride and an oxidation-reduction system. It is disclosed in "Journal of the Electrochemical Society," vol. 42, page 108 (1961).
In this photochemical cell, a metal electrode is covered with silver chloride, light is applied to the silver chloride electrode immersed in an aqueous solution containing Fe.sup.2+ ions to deposit silver, and the silver thus deposited is employed as the negative electrode while a platinum electrode is employed as the positive electrode to cause discharge to thereby obtain electrical energy. This photochemical cell is advantageous in that energy storage can be reaccomplished by applying light thereto after discharge. The energy storage is carried out by silver trapped in the AgCl lattice defects, which leads to the provision of a compact cell. However, it is still disadvantageous in that self-discharge occurs during periods of non-use, and the quantum yield of photo-decomposition is low (of the order of 2%). Thus, the amount of photo-decomposed silver is reduced by self recharge and, the amount of energy storage (charge) is reduced.
U.S. Pat. No. 3,114,658 discloses a method where a sheet of AgCl is employed as a film adapted to separate the positive pole from the negative pole in order to prevent the self-discharge of the above-described photochemical cell. However, this method cannot overcome drawbacks other than self-discharge.